Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been considered for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing faces of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the two half-cells. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof will synonymously refer to materials that undergo a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging). Transition metals and their coordination complexes can be particularly desirable active materials due to their multiple oxidation states.
Although flow batteries hold significant promise for large-scale energy storage applications, they have often been plagued by sub-optimal energy storage performance (e.g., round trip energy efficiency) and limited cycle life, among other factors. Certain factors leading to poor energy storage performance are discussed hereinafter. Despite significant investigational efforts, no commercially viable flow battery technologies have yet been developed.
Oxidation and reduction of the active materials in a flow battery are desirable electrochemical reactions, since they contribute to the battery's proper operation during charging and discharging cycles. Such reactions may be referred to herein as “productive reactions.”
In addition to desirable productive reactions, undesirable parasitic reactions can also occur within one or both half-cells of flow batteries and related electrochemical systems. As used herein, the term “parasitic reaction” will refer to any side electrochemical reaction that takes place in conjunction with productive reactions. Parasitic reactions can often involve a component of an electrolyte solution that is not the active material. Without being bound by any theory or mechanism, components within an electrolyte solution can achieve sufficient overpotential under some conditions to drive the occurrence of parasitic reactions. Stated alternately, the components of an electrolyte solution can be sufficiently reducing or sufficiently oxidizing under certain conditions to affect parasitic reactions. Parasitic reactions that can commonly occur in electrochemical cells containing an aqueous electrolyte solution are evolution of hydrogen and/or oxidation by oxygen. Hydrogen evolution, for example, can at least partially discharge the negative electrolyte of an electrochemical system while leaving the positive electrolyte unchanged. In addition, parasitic reactions can change the pH of an electrolyte solution, which can destabilize the active material therein in some cases. In non-aqueous electrolyte solutions, the electrolyte solvent can be oxidized or reduced in a parasitic reaction. Further, in both aqueous and non-aqueous systems, electrode materials and other surfaces can also undergo parasitic reactions (e.g., carbon or metal corrosion, separator oxidation, or the like) in some instances.
Discharge associated with parasitic reactions can decrease the operating efficiency and other performance parameters of a flow battery. In the case of a parasitic reaction that occurs preferentially in one half-cell over the other, an imbalance in state of charge can result between the negative and positive electrolyte solutions. The term “state of charge” (SOC) is a well understood electrochemical energy storage term that will refer herein to the relative amounts of reduced and oxidized species at an electrode within a given half-cell of an electrochemical system. Charge imbalance between the electrolyte solutions of a flow battery can lead to mass transport limitations at one of the electrodes, thereby lowering the round-trip operating efficiency. Since the charge imbalance can be additive with each completed charge and discharge cycle, increasingly diminished performance of a flow battery can result due to parasitic reactions.
Charge rebalancing of one or both electrolyte solutions can be conducted to combat the effects of parasitic reactions. Although various charge rebalancing techniques are available, they can be costly and time-consuming to implement. Determining the true concentration of oxidized and reduced active material species in an electrolyte solution can oftentimes itself be difficult, thereby adding a further difficulty to the charge rebalancing process.
In view of the foregoing, flow batteries configured to decrease the incidence of parasitic reactions and methods for mitigating parasitic reactions would be highly desirable in the art. The present disclosure satisfies the foregoing needs and provides related advantages as well.